1. Field of the Invention
This invention relates generally to a power transmission belt, more particularly to an adhesive system for tensile cord in a belt, and specifically to the use of isocyanate-terminated diene polymer to bond cord to elastomeric belt material.
2. Description of the Prior Art
V-belts and multi-V-ribbed belts in particular are conventionally employed for the transmission of peripheral forces in drive systems. Conventional belts have a flexible, generally elastomeric main belt body portion defining an undercord- or compression section, an overcord- or tension section and a tensile- or load-carrying section disposed between the undercord and overcord sections. The load-carrying section in turn generally includes a longitudinally extending highly resilient tensile member typically formed of one or more tensile cords. The generation of adhesion between the tensile cords and rubber compounds in dynamic products such as belts, tires, couplings, or hose is fundamental to their proper operation. Without significant adhesion, the product will fail to perform under dynamic conditions. An adhesive system may include many components or layers beginning with a fiber sizing applied to the fibers as they are spun, a primer typically applied to a yarn or greige cord or fabric or other fibrous reinforcement, an adhesive applied to and/or within the cord, and an overcoat applied to the treated cord bundle (or other fibrous reinforcement) to ensure compatibility with the surrounding rubber compound. The treated cord may finally be embedded in or surrounded by an adhesive- or gum-rubber composition generally formed from one or more rubber plies or layers making up the body of the belt or other dynamic rubber product.
A conventional treatment for polyester cord consists of three coatings: 1) a primer treatment of isocyanate or epoxy in organic solvent; 2) a resorcinol-formaldehyde-latex (“RFL”) treatment; and 3) a conventional overcoat adhesive based on a complex blend of chlorinated polymers, curatives, crosslinkers, adhesion promoters, film-formers, and/or acid scavengers, and the like such as one of the Chemlok® adhesives produced under that trademark by Lord Corporation and exemplified by U.S. Pat. No. 6,512,039. This is a complex series of treatments that are expensive, involve some environmentally unfriendly components, and require optimization for best performance. Examples of such three-step adhesive treatments are disclosed in U.S. Pat. Nos. 6,607,828 and 5,610,217. A variation used for aramid cord, disclosed in U.S. Pat. No. 5,246,735, includes a first polyurethane treatment based on hydroxyl- or epoxy-terminated liquid rubber and isocyanate-terminated liquid rubber either of which may be a diene rubber, followed by a conventional RFL treatment. Presumably, an overcoat adhesive would also be needed for many applications, such as for bonding to ethylene-alpha-olefin rubber compounds.
Steven K. Henning and Herbert Chao, Rubber World, Vol. 234, No. 7, p. 25 (October 2006), disclose the use of a thermoplastic polyurethane (“TPU”) resin, sold under the trade name Poly bd 2035 TPU by Sartomer Company, Inc., based on a polybutadiene-diol prepolymer, a diisocyanate, and a cross-linker or chain extender, as a co-curable adhesive tie layer between rubber compounds and a polyurethane component. Likewise, U.S. Pat. No. 4,774,142 discloses a coupling agent layer for bonding diene rubber to polyurethane plastic in tires, with the coupling agent formed of a polyenol polyurethane which is a reaction product of polybutadiene-diol, a diisocyanate, and a cross-linker or chain extender.
U.S. Pat. No. 6,660,376 discloses a hot-melt polyurethane adhesive for fabric bonding based on a prepolymer that is the reaction product of a polyol, a polyisocyanate, and a high-molecular-weight thermoplastic polyurethane lacking hard segments. The polyol may be a polybutadiene-diol, polyester, or polyether diol. U.S. Pat. No. 5,401,565 discloses for heat-bonding textiles a heat-adhesive layer comprising a hydroxylated polybutadiene resin, a free isocyanate, and a blocked isocyanate.
U.S. Pat. No. 4,535,121 discloses a polyurethane/epoxy adhesive system for flocking elastomers such as ethylene-propylene-diene elastomer (“EPDM”), the adhesive including an isocyanate-terminated prepolymer, which may be polybutadiene-based, and a reaction product of an aromatic diisocyanate with a polyfunctional epoxide.
Polyurea adhesives and coatings are known. Representative examples are provided by U.S. Pat. Nos. 5,183,877, and 5,759,695. The polyureas disclosed in the art are typically based on diamine- and/or diisocyanate-terminated polyethylene or propylene glycol and may include other hydrogen-containing material and/or amine-terminated chain extender including polyether or polyester polyol.